Optical amplifiers may be utilized in optical communication networks, especially long-haul networks of lengths between 600 kilometers and 10,000 kilometers, to compensate for optical losses. Optical amplifiers amplify an input optical signal without converting it into electrical form. They include rare earth doped fibers such as erbium doped fiber amplifiers (EDFAs) and Raman amplifiers.
An EDFA operates by passing an optical signal through an erbium-doped fiber segment, and “pumping” the segment with light from another source such as a laser. Raman amplification occurs throughout an optical transmission fiber when the transmission fiber is pumped at an appropriate wavelength or wavelengths. Gain is then achieved at a longer wavelength through the process of Stimulated Raman Scattering. The difference between the Raman amplifier pumped wavelength and the associated amplified wavelength spectrum at the longer wavelength is referred to as a “Stokes shift.” The Stokes shift for a typical silica fiber is approximately 13 THz. Hence, Raman amplifiers provide amplification of an optical signal without the need for a specially doped fiber, such as used in an EDFA.
Signal degradation in optical communication systems due to noise is largely unavoidable. There are various types of noise such as thermal noise, shot noise, modal noise, and mode partition noise. In optical systems utilizing optical amplifiers, each optical amplifier is also a source of noise. The noise figure F for an optical amplifier gives an indication of the degradation in a transmitted signal owing to that amplifier, and is based on a ratio of the input signal-to-noise ratio to the output-signal-to-noise ratio. Thus, an amplifier exhibiting a relatively large noise figure can negatively affect signal detection and system reliability.
In addition, some optical systems, such as WDM and DWDM systems, transmit a plurality of channels over a range of wavelengths. In such systems, any “ripples” due to an uneven noise figure can also negatively affect signal detection and system reliability. Such “ripples” may also be cumulative if successive optical amplifiers exhibit similar noise figures.
Accordingly, there is a need for an optical amplifier configuration and method that overcomes the deficiencies of the prior art by providing a desired noise figure across a transmitted range of wavelengths.